Electroluminescent silicon carbide diode with sharply peaked light emission from the edge of the junction



Sept. 22, 1970 L. R. KOLLER ET AL 3,530,324

ELECTROLUMINESCENT SILICON CARBIDE DIODE WITH SHARPLY PEAKED LIGHT EMISSION FROM THE EDGE OF THE JUNCTION Filed May 16, 1967 4 Sheets-Sheet 1 n relafively Nonspareni I00 cm A between 95 and "2* Fig.!

Sept. 22, 1970 L. R. KOLLER ETAL 3,

ELECTROLUMINESCENT SILICON CARBIDE DIODE WITH SHARPLY PEAKED LIGHT EMISSION FROM THE EDGE OF THE JUNCTION Filed May 16, 1967 4 Sheets-Sheet 2 relatively transparent 5 3 l 0 E l 8 .g 3- I m II II 1 half width =.-OO|5 cm |5,u. .2 l-- E 2- "holTwidth" 30, c?

l O 0| .02 -x Sept. 22, 1970 KOLLER ETAL 3,530,324

ELECTROLUMINESCENT SILICON CARBIDE DIODE WITH SHARPLY PEAKED LIGHT EMISSION FROM THE EDGE OF THE JUNCTION Filed May 16, 1967 4 Sheets-Sheet 5 Relative Brightness X-cms Fig. 3

Sept. 22, 1970 R KQLLER ET AL 3,530,324

ELEGTROLUMINESCENT SILICON CARBIDE DIODE WITH SHARPLY PEAKED LIGHT EMISSION FROM THE EDGE OF THE JUNCTION Filed May 16, 1967 4 Sheets-Sheet 4 FILM 2r ADVANCING MECHANISM 26 PHOTOGRAPHIC FILM.

-|s l \L|GHT FROM DIODE United States Patent 3,530,324 ELECTROLUMINESCENT SILICON CARBIDE DI. ODE WITH SHARPLY PEAKED LIGHT EMISSION FROM THE EDGE OF THE JUNCTION Lewis R. Koller, Cambridge, and Allan S. Miller, Wellesley, Mass., assignors, by mesne assignments, to Norton Research Corporation Filed May 16, 1967, Ser. No. 638,868 Int. Cl. G01d 9/42; Gllb 7/12; Hb 33/16 US. Cl. 313-108 4 Claims ABSTRACT OF THE DISCLOSURE An electroluminescent diode, such as a silicon carbide diode, has one edge cut so that it makes an angle of between about 5 and 22 to the normal to the plane of the junction. Accordingly, some of the light which would normally escape from the edge is internally reflected and the effective width of the light emitted from the junction is considerably narrower than would be the case if the edge were perpendicular to the junction. When the edge of the junction is held in close proximity to motion picture film, it can be utilized for recording sound on the film in a sound motion picture camera.

The present invention is particularly directed to an improved electroluminescent junction diode, particularly a silicon carbide diode which is useful for recording a high density of data on a photographic film, such as a sound track on a motion picture film.

The principal object of the present invention is to provide an improved electroluminescent junction diode having a very narrow emitted beam of light.

Another object of the invention is to provide an improved device for recording data on film employing such an electroluminescent diode.

These and other objects of the invention will be obvious and in part appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed discussion thereof taken in connection with the accompanying drawing in which:

FIG. 1 is a diagrammatic, schematic representation of a silicon carbide diode embodying the present invention; and

FIG. 2 is an enlarged diagrammatic representation of a section of the crystal of FIG. 1;

FIG. 3 is a series of graphs showing the intensity of light emitted at the surface of the diode as a function of distance from the junction with distance along the diode face as a parameter;

FIG. 4 is a plot of the relative brightness of the light along the diode face as a function of distance from the junction; and

FIG. 5 is a schematic, diagrammatic representation of the diode as used in conjunction with a sound motion picture camera.

The invention is particularly concerned with an improved electroluminescent junction diode, such as silicon carbide, having a beam of emitted light which is particularly useful for recording high density of data on photographic film, for example, for recording sound track on motion picture film. For convenience the invention is initially described in connection with the use of the preferred siliconcarbide junction diode. The silicon carbide juncton diode of the present invention has a light-emitting face which is generally perpendicular to the plane of the junction. One side of the junction is relatively opaque to light generated at the junction and the other side is relatively transparent to such light, at least in the region ice of the junction. 0n the opaque side the absorption coeflicient a for light in the wavelength of the emission should be 250 cm. or greater where I=I e-ad; I is the incident intensity; I is the intensity after absorption in a media of absorption coeflicient 0c CH1. 1, which is d cm. thick. On the transparent side the absorption coefficient a should be less than 100 cmf The light-emitting face of the diode is ground or cut so as to make an angle between about and 112 (as measured through the relatively transparent side) with respect to the plane of the junction. Accordingly, light generated in the junction is substantially absorbed in the relatively opaque side of the junction, but is transmitted through the transparent side. However, the width of the beam being emitted from the face is narrowed because of the fact that any light approaching the face at an angle of incidence (the angle between the light beam and the perpendicular to the surface) which exceeds the critical angle of about 22 is totally internally reflected and does not escape from the crystal. Thus, for example, a crystal which would normally have (with a perpendicular face) an emitted beam having a half width of 30 microns will, when ground so that the edge makes an angle of with respect to the plane of the junction (as measured through the relatively transparent side), have a light beam with a half width of only 15 microns.

In order that the invention may be more fully understood, reference should be had to the highly diagrammatic, schematic representation shown in FIG. 1. In this drawing, the junction diode is shown as comprising an n-type crystal 10 which is relatively transparent and is superimposed on a p-type material 12 the composite structure being in the form of a single crystal and having a junction which is indicated at 14. Normally such a crystal will have a face 16 which is perpendicular to the plane of the junction, this being the normal cleavage, cutting or sawing line. At the righthand side of FIG. 1 the normal face of the diode is shown in dotted lines at 16'. However, the light-emitting face is ground as at 18, thereby making a slight angle to the normal. Accordingly the angle A (as measured between the plane of the junction and the edge 18) is between about 95 and 112. With this change the effective width of the light emitted from the junction immediately adjacent the junction is considerably narrower than would be the case with light emitted from the edge which is normal to the plane of the junction.

Referring now to FIG. 2, there is illustrated an enlarged diagrammatic representation of a section of the crystal of FIG. 1 wherein the light emitting face 18 is shown as being ground to an angle ,8 with respect to the normal 16' to the junction 14. As can be seen the angle A equals 90 +,8. Assuming that the plane of the junctions is 0 the light intensity I (or illumination) at the crystal face 18 can be calculated along the direction ox for a series of luminescent elements along the plane of the junction oh. The light will be attenuated in the crystal by absorption and because of the inverse square law. At any point x, the distance h x from a luminescent element k can be calculated from trigonometric relations, since the distances oh and 0x the angle between them are known. Accordingly, the incremental illumination AI which is due to light emitted from h, and which escapes from the diode surface at x, is

3 arbitrarily chosen at 0.005 cms. from the x axis. The calculation was repeated for a number of points along the x axis. The curves for the relation between I and x are shown in FIG. 3. As we move out in the x direction the angle of incidence (the angle between the light beam and the perpendicular to the surface) increases. When it reaches the critical angle of about 22 the light is totally internally reflected and does not escape from the crystal. The critical angle is determined by the equation C=arc sin l/n where n is the index of refraction of the material at the wavelength of the emitted light.

It should be pointed out that this relationship holds true only when the light is emitted into air which has an index of refraction of 1. A similar relation holds for each luminescent element along 0h. Accordingly each of the curves of FIG. 3 is effectively terminated at the point indicated by the arrow. Although the more remote elements illuminate regions further along 0x their contribution becomes small because of their greater distance. The total illumination along 0x was calculated by adding the ordinates of the curves of FIG. 3. This curve is shown as the full line in FIG. 4. In making this calculation the angle 5 at which the crystal was sliced was taken as As is shown in FIG. 4, the half width of the curve is approximately 0.0015 cm.

In making this calculation the index of refractionw as taken as 2.5 and the absorption coeflicient of the n and p materials as 5 and 500, respectively. The absorption in the p material is so high that the approximation was made that no light escapes on this side. The reflection coefiicient Was assumed to be constant until the critical angle was reached when it became 100%, and the cosine was neglected. These two effects would not greatly change the shape of the light distribution curve.

The result of slicing the crystal at the angle ,8 to normal 16 is to decrease the width of the beam. If this angle is 22 the half width is 0, at 10 about 1. as plotted and at 0 about 301A. Thus, the beam at the face of the crystal may be made any width from 30a to 0. (1,l.L 10 cm.). At the same time, the total light flux is reduced in ap proximately the same ratio as the beam width. In FIG. 4 the dotted line curve illustrates the calculated curve of light intensity when the angle 8 is 0 and the half width iS 3011..

In the above discussion, reference has been made to half width. This normally means one-half the width of the calculated curve of light distribution at the halfheight, assuming a symmetrical curve on both sides of the light generating plane. However, in the preferred diodes, one-half of the crystal is essentially opaque to the emitted light. In this case, if the light-emitting area is very close to the electrical junction, half of the light which would otherwise escape from the diode is internally absorbed, and, accordingly, the light at the surface will have an asymmetric intensity distribution. As a result, the fhalf width may be nearly as large as the actual width.

In the above discussions of light emission, the face of the diode is assumed to have a polished surface. If the surface is not polished but is perfectly scattering, there will be many points on the surface of the edge far removed from the plane of the junction which will, on a microscopic scale, present a surface which is nearly normal to a light bea-mwhich makes an angle much greater than the critical angle. Accordingly, this widely divergent light beam can escape from the diode face.

In order to more fully understand the use of the novel diode in a sound movie camera, reference should be had to FIG. 5 which is a highly diagrammatic, schematic representation. In this FIG. 5 the photographic film is shown at 20 as being advanced by a constant speed film advancing mechanism 21 past the light-emitting edge 18 of the diode described in FIG. 1. Suitable leads 22 and 24 connect the diode to a sound system 26 which provides a sound-modulated electrical current for varying the light output from the diode.

In a preferred embodiment of the invention, the diode is prepared in accordance with the following nonlimiting example.

EXAMPLE 1 of about 10 with respect to the horizontal and plotted while at this angle. Then the edge 18 was ground so that it made an angle of about 100 with respect to the junction as measured through the relatively transparent n section. This crystal was then arranged to be mounted in a 16 mm. motion picture camera with the edge 18 in close proximity (less than about .5 mil) to the film.

Since certain changes can be made in the above apparatus and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electroluminescent silicon carbide junction diode which generates light in the junction and has a lightemitting face transverse to the plane of the junction, one side of the junction being relatively opaque to light generated in the junction and the other side being relatively transparent to such light, at least in the region of the junction, the light-emitting face of the junction making an angle with respect to the plane of the junction, as measured through said relatively transparent side, of between and 112.

2. The diode of claim 1 wherein the relatively opaque side of the junction has an absorption coefficient of at least 200 cm.- and the relatively transparent side of the junction has an absorption coeflicient of less than cm.

3. The diode of claim 2 wherein the relatively opaque side of the junction has an absorption coefficient of about 4. The diode of claim 2 wherein the relatively transparent side of the junction has an absorption coefiicient of about 5.

References Cited BERNARD KONICK, Primary Examiner 'R. F. CARDILLO, JR., Assistant Examiner US. Cl. X.R. 

